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Relay to control window air unit - first attempt failed (seeking advice) Answered

I'm working on a thermostat project to control window AC units (which I will create an instructable for when it works). The need is to switch on and off an AC unit. The unit runs on 240 VAC (split phase) and the technical documentation says to put it on a circuit with a 20 AMP circuit breaker. I will be controlling the relay with 5 volts from an Arduino (although I suppose I can use an additional small relay to control the larger relay with a higher control voltage, if necessary... the less moving parts the better.)

I already tried with a 20 AMP Solid State Relay (SSR) from Omron, but it failed. The problem is that, due to the voltage drop in an SSR, they get really hot. The hotter they get, the less current they are able to switch. The other problem is that SSRs fail with the circuit closed or partially closed. This is not ideal. You wouldn't want to come back to your place after a vacation and find your AC had been running constantly the whole time...

I am thinking I need to use an electromechanical relay, but there are so many options on Mouser that I'm feeling a bit overwhelmed! Can someone please help point me in the right direction or suggest a good relay to use?


Why are you trying to control the unit directly from the mains voltage going into it. Why not add the controls to your existing thermostat. They typically run off of 24VAC. Unless this is a much older system.

You could also just spend $30 on a digital unit that you can program with multiple temp settings for different times of the day.

The main reason is that this is a wall unit with an integrated thermostat. I've also been asked specifically not to modify the unit itself. That would, unfortunately, preclude me from using some low-voltage relays to trigger the system on and off.

What is the purpose of taking control of the A/c? You want to be able to set the times is it running or have it remotely turned on or off? A/Cs usually have logic built in that doesn't like it when they are power cycled frequently so rapid starts and stops might interfere with getting the room to a stable temperature, assuming the thermostat is still working and that is not what you want to affect. Off the shelf timer units aren't made to handle appliance size loads.

In this case the AC is not doing a great job controlling the temperature. I also want to specifically control for humidity by allowing a certain amount of over-cooling.

To protect the compressor, I have a built-in timer in my code to make sure the AC will not cycle back on unless it has been off for at least 3 minutes. I also have temperature hysteresis so I can vary the amount of degrees the temperature is allowed to fluctuate.

I'm hoping to find a good non solid-state relay that would work well for this project.

The the unit has a physical knob right. Well use the Arduino and a motor shield so that you can control a motor that turns the knob.

That is definitely an interesting and most likely better way of doing it than what I originally was planning. The other thing I like about your idea is that it would give me the added ability of controlling fan speed which would be helpful for controlling humidity and maintaining a more consistent temperature.

I'll have to do some research... what I am not sure about is how best to mount the motors, but I bet there is an instructable with guidance. I love this site!

Any control knobs on the unit will either have a set screw or can be pulled straight off. Then a simple shaft coupler can be used to link the motor to the controls.

Perfect! I think I will likely pursue this way of doing things.

Better post this in the specific "Answers" section of the site.

Thanks Steve. I really was not sure where to post this.

Thanks again Steve! I did get a very good response to the question. Appreciate your pointing me in the right direction.

Here's a link to the question and answer in case anyone comes across this post and has the same question:

Hope this is the right forum to put this in, if not please let me know or moderators feel free to move it to where it belongs.